have you checked to make sure you can source all the parts you need?.
Yes, all the bits are available still. The MM5450 is the dearest part and is available for $3.80 ea for a tube of 26. Remember I want to fit it on the back ... so I'd use a PLCC44.
The switches are ~$1.91 each .... and the last tube of displays I bought cost $2.39 each.
The other two ICs are a 74LS145 (1 of 10 Decoder/Driver) and a CD4093 (Quad 2 Input NAND Gate)
Have you also considered completely redesigning the board?
Mmmm ... hang on ... by redesigning the board do you mean redesign the board ... or redesign the circuit ??
Certainly I want to redesign the layout on the board ...
... but redesigning the circuit ... I did want to avoid that ... but I have considered it.
All these great questions have forced me to think it through even further.
I guess if I used a micro to read the input for the existing display driver ... and multiplexed the outputs to the 7 segment displays ...
... and then use further inputs to read the keyswitches and send key presses to the rear board micro ... and lastly provide outputs to drive the 10 individual leds.
Then I could certainly leave out the two separate ICs.
I can't see it being all that much less complicated track layout wise ... still have to have tracks to all the displays and all the keys.
Again ... thinking about it some more ...
Multiplexing the 4 x displays would cut it down from 32 traces up to that area ... down to just 12 ... mmm!
do you know what the connections are from the front board to the back one?
Yes, pretty much ... there's 11 of them.
There's 5 volts and GND for the board ...
Then DATA and CLK_IN for the M5450 ... I need to read up on the datasheet to see how it's formulated.
At present there's 3 x BCD lines to the 74LS145 Decoder for reading the key switches ...
... and helping switch the 10 x individual leds (split into 2 groups)
Plus 2 more Inputs to select which of those 2 groups to turn on ...
Then 2 output lines back to the rear board micro to tell it which switch has been pressed.